Node capable of saving a third-layer operation

ABSTRACT

In a node having first, second, and third layers, a packet (or a cell) is mapped in the first layer. The first layer judges whether the packet (or the cell) is to be dropped at the node or to be hopped to a next node. The first layer transmits the packet to the third layer through the second layer when the first layer judges that the packet is to be dropped at the node. The first layer transmits, when the first layer judges that the packet is to be hopped to the next node, the packet to the next node by making the packet cut through the first layer.

BACKGROUND OF THE INVENTION

This invention relates to a cut-through transmission apparatus and acut-through transmission method for use in a node included in acommunication network.

In recent years, the internet traffic is rapidly increased. The internettraffic is carried by IP (Internet Protocol) packets which aretransferred under control of a router. Following the rapid increase ininternet traffic, there is a growing demand for improvement in functionand performance of the router. Since most of transferred data areimportant data for business use, a strict demand is imposed upon thereliability, the quality, and the security. On the other hand, therearises an increasing demand for a virtual private network, such as aninternet VPN service, connecting a plurality of sites or nodes.

In order to meet the above-mentioned demands, one approach is to makethe router have a high performance and a full of additional functions.However, this approach has a limit. There remains a problem how torealize the above-mentioned demands in an IP packet network inassociation with an existing transmission network.

As the existing transmission network, there are known an SDH(Synchronous Digital Hierarchy) network, an ATM (AsynchronousTransmission Mode) network, a WDM (Wavelength Division Multiplexing)network, and a PDH (Plesiochronous Digital Hierarchy) network. Byintegrating the existing transmission network and the IP packet networktransmitting the internet traffic and by complementing theircharacteristics with each other, it is expected to achieve an improvedsystem having more efficient transmission characteristics.

Consideration will be made about conventional network structures forLAN-to-LAN connection.

Referring to FIG. 1A, four nodes A through D are connected through aplurality of SDH paths 100 in a point-to-point connection. Specifically,the SDH paths 100 as private lines are provided between the nodes A andB, between the nodes A and C, between the nodes A and D, between thenodes B and C, between the nodes B and D, and between the nodes C and D,respectively. This network structure called a mesh type isdisadvantageous in that a large number of SDH paths (private lines) arerequired.

Referring to FIG. 1B, four nodes E, F, G, and H are connected through anIP packet network of a hop-by-hop connection. Specifically, a pluralityof SDH paths 101 as private lines are provided between the nodes E andF, between the nodes F and G, between the nodes G and H, and between thenodes H and E. In addition, each of the nodes E, F, G, and H has arouter function to execute a routing operation.

Referring to FIG. 2, a router 102 performs the routing operation as athird-layer operation. Specifically, the router 102 individuallyidentifies all packets flowing into the router 102 to judge whether eachpacket is to be dropped at this node or to be hopped to a next node.

In order to execute the routing operation, the router 102 is required tohave a number of functions. Thus, a heavy load is imposed upon therouter 102.

Specifically, the router 102 is required to have a large routing tableincluding routing information for all packets flowing into the router102 and to have a high performance so as to search such a large routingtable.

In addition, in order to interface the SDH paths (the private lines) inthe form of DS-1 (Digital Signal Level 1), the router 102 is required tohave a path terminating function of terminating VT (Virtual Tributarylevel) 1.5.

Since all of the packets are sent to a third layer after subjected tothe above-mentioned operations, a processing delay is produced also forthose packets which are to be hopped to the next node and need not beprocessed at this node. This results in significant degradation of anetwork performance as a whole.

Thus, packet transfer in the above-mentioned IP packet network isdisadvantageous in that the third-layer operation, i.e., the routingoperation inevitably gives the heavy load.

For example, Japanese Unexamined Patent Publication (JP-A) No.H10-136016 (136016/1998) discloses a packet transfer control methodwhich is capable of shortening a time required for a router to create aprivate cut-through path for a particular end flow.

The operation of the above-mentioned packet transfer control method isas follows. It is assumed that, in order to transfer a packet flowdefined by a relatively abstract (general) condition such as adestination network address, a first cut-through path is preliminarilyestablished from a first router to a second router which is not adjacentto the first router. In this state, it is assumed that a secondcut-through path is required to be established to transfer a packet flowdefined by a more specific (detailed) condition such as asource/destination address pair and a destination port number. In thisevent, control messages for establishment of the second cut-through pathare exchanged between the first router and the second router as anendpoint of the first cut-through path. Thus, the second cut-throughpath is established by the use of the first cut-through path.

In the above-mentioned packet transfer control method, cut-throughconnection is established by the router. As a result, an operation loadimposed upon the router is not reduced.

On the other hand, Japanese Unexamined Patent Publication (JP-A) No.H10-294737 (294737/1998) proposes a packet transfer apparatus which iscapable of reducing a delay which is produced upon start of transfer ofa best-effort flow and required to establish a connection between thepacket transfer apparatus and another packet transfer apparatus.

In the packet transfer apparatus, each router containing a switchmonitors a time duration of the flow. If the flow continues for a longtime, a cut-through connection is established by switching and assignedto the flow. If the flow continues for a much longer time, a short-cutconnection using an ATM connection is established and assigned to theflow. Thus, a basic router is avoided.

In the above-mentioned packet transfer apparatus also, cut-throughconnection is established by the router, like in Japanese UnexaminedPatent Publication (JP-A) No. H10-136016. As a result, the operationload imposed upon the router is not reduced.

Japanese Unexamined Patent Publication (JP-A) No. H09-172457(172457/1997) proposes a packet transmission node apparatus, which iscapable of establishing cut-through connection selectively for a trafficexpected to have a relatively large amount of communication afterestablishment of the cut-through connection.

In the packet transmission node apparatus, a node which can be astarting point or an end point of the cut-through connection refers to,before transmission or after reception of a packet, not only informationof a network layer of the packet but also at least one of sourceinformation and destination information of a transport layer. If it isjudged as a result of the reference that establishment of thecut-through connection is worthwhile, initiation of connectionestablishment is triggered by the packet.

In the packet transmission node apparatus, the cut-through connection isestablished not in the router but in the node. Therefore, the load uponthe router is reduced. However, since the various kinds of informationmust be referred to as mentioned above, packet transmission inevitablyrequires a complicated operation.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a cut-through transmissionapparatus (or a node) which is capable of remarkably saving athird-layer operation, i.e., a routing operation.

It is another object of this invention to provide a cut-throughtransmission method which is capable of remarkably saving a third-layeroperation, i.e., a routing operation.

According to this invention, there is provided a node comprising first,second, and third layers, wherein:

-   -   a packet is mapped in the first layer;    -   the first layer judging whether the packet is to be dropped at        the node or to be hopped to a next node;    -   the first layer transmitting the packet to the third layer        through the second layer when the first layer judges that the        packet is to be dropped at the node.

The first layer transmits, when the first layer judges that the packetis to be hopped to the next node, the packet to the next node by makingthe packet cut through the first layer.

According to this invention, there is also provided a node comprisingfirst, second, and third layers, wherein:

-   -   the second layer judges, without terminating the first layer,        whether a packet supplied from the first layer is to be dropped        at the node or to be hopped to a next node;    -   the second layer transmitting the packet to the third layer when        the second layer judges that the packet is to be dropped at the        node.

The second layer transmits, when the second layer judges that the packetis to be hopped to the next node, the packet to the next node by makingthe packet cut through the second layer.

According to this invention, there is also provided a node comprisingfirst, second, and third layers, wherein:

-   -   the second layer transmits, when a packet supplied from the        first layer is not to be dropped at the node, the packet to a        next node by making the packet cut through the second layer        without terminating the first layer;    -   the second layer transmitting the packet to the third layer when        the packet is to be dropped at the node.

According to this invention, there is also provided a node comprisingfirst, second, and third layers, wherein:

-   -   if packets to be dropped and not to be dropped at the node are        both contained in a transmission path, the second layer monitors        all packets in the transmission path to transmit, when the        packet is not to be dropped at the node, the packet to a next        node by making the packet cut through the second layer and to        transmit the packet to the third layer when the packet is to be        dropped at the node.

According to this invention, there is also provided a transmissionapparatus comprising:

-   -   a time slot extracting section for converting an input optical        signal supplied through a first point into an input electric        signal and for selecting among time slots in a transmission path        of the input electric signal a particular time slot which        includes a packet to be dropped at a second point;    -   a drop packet extracting section for monitoring all packets in        the particular time slot selected in the time slot extracting        section to identify whether or not each packet is to be dropped        at the second point;    -   an add packet inserting section for packet-multiplexing the        packet not to be dropped at the second point and a packet        inserted at the second point to produce a packet-multiplexed        packet; and    -   a signal transmitting section for inserting into an appropriate        time slot of the transmission path the packet-multiplexed packet        to be sent to a third point, converting an output electric        signal including the transmission path into an output optical        signal, and delivering the output optical signal to the third        point.

According to this invention, there is also provided a transmissionmethod carried out in a node comprising first, second, and third layers,comprising the steps of:

-   -   judging, in the first layer where a packet is mapped, whether        the packet is to be dropped at the node or to be hopped to a        next node; and    -   transmitting in the first layer the packet to the third layer        through the second layer when the first layer judges that the        packet is to be dropped at the node.

The transmission method may further comprise the step of:

-   -   transmitting in the first layer, when the first layer judges        that the packet is to be hopped to the next node, the packet to        the next node by making the packet cut through the first layer.

According to this invention, there is also provided a transmissionmethod carried out in a node comprising first, second, and third layers,comprising the steps of:

-   -   judging in the second layer, without terminating the first        layer, whether a packet supplied from the first layer is to be        dropped at the node or to be hopped to a next node; and    -   transmitting in the second layer the packet to the third layer        when the second layer judges that the packet is to be dropped at        the node.

The transmission method may further comprise the step of:

-   -   transmitting in the second layer, when the second layer judges        that the packet is to be hopped to the next node, the packet to        the next node by making the packet cut through the second layer.

According to this invention, there is also provided a transmissionmethod carried out in a node comprising first, second, and third layers,comprising the steps of:

-   -   transmitting in the second layer, when a packet supplied from        the first layer is not to be dropped at the node, the packet to        a next node by making the packet cut through the second layer        without terminating the first layer; and    -   transmitting in the second layer the packet to the third layer        when the packet is to be dropped at the node.

According to this invention, there is also provided a transmissionmethod carried out in a node comprising first, second, and third layers,comprising the step of:

-   -   monitoring in the second layer, if packets to be dropped and not        to be dropped at the node are both contained in a transmission        path, all packets in the transmission path to transmit, when the        packet is not to be dropped at the node, the packet to a next        node by making the packet cut through the second layer and to        transmit the packet to the third layer when the packet is to be        dropped at the node.

According to this invention, there is also provided a transmissionmethod comprising the steps of:

-   -   converting an input optical signal supplied through a first        point into an input electric signal;    -   selecting among time slots in a transmission path of the input        electric signal a particular time slot which includes a packet        to be dropped at a second point;    -   monitoring all packets in the particular time slot selected in        the selecting step to identify whether or not each packet is to        be dropped at the second point;    -   packet-multiplexing the packet not to be dropped at the second        point and a packet inserted at the second point to produce a        packet-multiplexed packet;    -   inserting into an appropriate time slot of the transmission path        the packet-multiplexed packet to be sent to a third point,    -   converting an output electric signal including the transmission        path into an output optical signal; and    -   delivering the output optical signal to the third point.

An ATM cell may be used instead of the packet.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1A and 1B are schematic diagrams showing existing LAN-to-LANconnections of a mesh type and a hop-by-hop type, respectively;

FIG. 2 is a schematic diagram showing an operation at a node in theexisting hop-by-hop connection illustrated in FIG. 1B;

FIG. 3 is a schematic diagram showing a specific example of a networkusing a cut-through transmission apparatus or a cut-through transmissionmethod according to this invention;

FIG. 4 is a schematic diagram showing a cut-through type node using thecut-through transmission apparatus of this invention;

FIG. 5 is a schematic diagram showing a specific example of thecut-through type node illustrated in FIG. 4;

FIG. 6 is a block diagram showing the structure of the cut-throughtransmission apparatus of this invention;

FIG. 7 is a schematic diagram showing a ring to which the cut-throughtransmission apparatus of this invention is applied;

FIG. 8 is a schematic diagram showing the ring illustrated in FIG. 7applied to a multi-tenant building; and

FIG. 9 is a schematic diagram showing an access network applied to thering illustrated in FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Herein, the term “cut-through” represents a connection in which theoperation in the first layer is omitted by some means even if ahop-by-hop connection is normally required, i.e., a connection in whichpacket transfer is carried out from one logical network to anotherlogical network only by the operation in a layer lower than the firstlayer.

In the cut-through transmission apparatus or the cut-throughtransmission method according to this invention, judgement about whethera packet is to be dropped at a current node or to be hopped to a nextnode and an operation following the judgement are not carried out in thethird layer individually for all packets. Instead, the judgement and theexecution are carried out in the first layer where the packet is mapped(“cut-through 1” which will later be described). Alternatively, thejudgement and the operation are carried out in the second layer withoutterminating the first layer (“cut-through 2” which will later bedescribed).

Referring to FIG. 3, a plurality of points are labelled A, B, C, D, E,F, G, H, I, J, and K. X Corporation has four offices at the points B, D,E, and I. A private line (x) SDH path 10 passes through the points B, A,D, C, E, H, and I. Thus, the private line (x) SDH path 10 passes throughall of the four offices of X Corporation at the points B, D, E, and I.On the other hand, Y Corporation has two offices at the points A and F.A private line (y) SDH path 11 passes through the points A, D, C, E, H,G, and F. Thus, the private line (y) SDH path 11 passes through the twooffices of Y Corporation at the points A and F. Furthermore, ZCorporation has two offices at the points J and K. A private line (z)SDH passes through the points J, E, and K. Thus, the private line (z)SDH path 12 passes through the two offices of Z Corporation at thepoints J and K.

The private line (y) SDH path 11 is a private line connected in apoint-to-point connection of an existing type. Therefore, the privateline (y) SDH path 11 can not be accessed, for example, from the point Dor E as an intermediate point.

On the other hand, the private line (x) SDH path 10 is a private lineestablished according to this invention. The private line (x) SDH path10 can be accessed also at the point D or E as the intermediate point.

According to this invention, packets of Y Corporation and Z Corporationcan be cut through at the point E on the private line (y) SDH path 11and the private line (z) SDH path 12 because these packets need not bedropped at the point E. Similarly, packets transferred from the point Dto the point I can be cut through at the point E on the private line (x)SDH path 10 because these packets need not be dropped at the point E.Hereinafter, the private line (x) SDH path or the like will be referredto as a shared SDH path.

In this case, the private line (x) SDH path 11, the private line (y) SDHpath 11, and the private line (z) SDH path 12 can coexist in a samenetwork.

Thus, in the cut-through transmission apparatus or the cut-throughtransmission method according to this invention, packets to be hopped toa next node are cut through in the first layer or the second layer.Therefore, only those packets to be dropped at this node are sent to thethird layer. Thus, it is possible to considerably save the third-layeroperation, i.e, the routing operation.

In case where the packets are cut through in the second layer, thecut-through is carried out without terminating the first layer.Therefore, a large amount of operation required to terminate the firstlayer (for example, pointer processing upon termination of VT1.5)becomes unnecessary. As a result, it is possible to considerably savefunctional operations.

The packets to be cut through are processed in the first or the secondlayer. In addition, the above-mentioned saving in functional operationsmakes it possible to minimize the delay. Thus, an end-to-end networkperformance is remarkably improved.

The cut-through inhibits the communication from being accessed at thatpoint. Therefore, the security is assured.

Referring to FIG. 4, description will be made about a cut-through nodeusing a cut-through transmission apparatus according to a firstembodiment of this invention.

The cut-through node of the first embodiment is called a “cut-through 1”node.

In this embodiment, each of a plurality of SDH paths 20 is taken as asingle unit for which a closed user group (CUG) is preliminarily formed.

In FIG. 4, an upper one of the SDH paths 20 does not carry any packet tobe dropped at the cut-through node. In this event, the SDH path 20 isnot terminated and packets on the SDH path 20 are made to pass throughto a next node (cut-through 1).

In this method, not only the termination of the SDH path (for example,VT1.5) but also the operation in a second layer 23 or a third layer 24(FIG. 5) become unnecessary. Therefore, it is possible to save aconsiderable amount of functions.

Furthermore, a packet delay at this node includes no more than a delayrequired for the packets on the SDH path 20 to pass through a firstlayer 22. Thus, the packets can pass through this node without anysubstantial delay.

Turning back to FIG. 3, the cut-through node will be described inconjunction with the specific example.

At the point E, the packets of Y Corporation and Z corporation need notbe dropped. Therefore, at the point E, the private line (y) SDH path 11or the private line (z) SDH path 12 need not be subjected to anyoperation at all. The cut-through is executed by the private line (x)SDH path 10 alone.

Referring to FIG. 5, the private line (x) SDH path 10 has a throughputof n x VT1.5 (n being an arbitrary natural number corresponding to anecessary band). For each unit of n x VT1.5, the private line (y) SDHpath 11 or the private line (z) SDH path 12 is subjected to “cut-through(pass-through) 1”.

In this case, it is only necessary to terminate a section overhead(SOH), a line overhead (LOH), and a path overhead (POH) as depicted by areference numeral 25. Termination of VT1.5 is unnecessary. Therefore,the cut-through can be realized with a very simple structure.

Turning back to FIG. 4, description will be made about a cut-throughnode using a cut-through transmission apparatus according to a secondembodiment of this invention.

The cut-through node of this embodiment is called a “cut-through 2”node.

In the cut-through 2, if a particular SDH path 20 contains packets to bedropped and not to be dropped at this node, all packets contained in theSDH path 20 are monitored in the second layer 23. The packets not to bedropped are cut through in the second layer 23 to the next node.

In the cut-through 2 also, the SDH path 20 is not terminated and cutthrough to the next node in the manner similar to the cut-through 1, asdepicted by a dotted line in FIG. 4.

In this embodiment, the cut-through is possible without requiring thefunction of terminating the SDH path (for example, VT1.5). In addition,the operation in the third layer 24 (FIG. 5) is not necessary at all.Thus, a considerable amount of functions can be saved.

A packet delay at this node includes no more than a delay required forthe packets to pass through the second layer 23. Therefore, the packetscan pass through this node with a relatively small delay.

Referring again to FIG. 3, the cut-through node will be described inconjunction with the specific example.

Among a group of packets sent through the private line (x) SDH path 10,those packets sent from the point D to the point I need not be droppedat the point E. Therefore, those packets are cut through in the secondlayer 23 without being sent to the third layer 24.

In FIG. 5, the SDH path 10 has a throughput of n x VT1.5 (n being anarbitrary natural number corresponding to a necessary band). In the SDHpath 10 of n x VT1.5, only those packets to be dropped at this node aresubjected to “cut-through (packet through) 2”.

In this case also, it is only necessary to terminate the sectionoverhead (SOH), the line overhead (LOH), and the path overhead (POH) asdepicted by the reference numeral 25, in the manner similar to the“cut-through 1”. The termination of the VT1.5 is unnecessary. Therefore,the cut-through is realized with a very simple structure.

In the example illustrated in FIG. 5, an input signal is decomposed intoa level of VT1.5. At that level, necessary time slots are selected.Instead of VT1.5, use may be made of VT2, VT3, VT6, STS-1, STS-3(STM-1), STS-12 (STM-4), STS-48 (STM-16), or STS-192 (STM-64).

The whole of the necessary band can includes a multiple of a single sortof band. Alternatively, use may be made of a mixture of VT1.5 and STS-1.

Furthermore, in addition to a variety of different sorts of bands in theSDH path as mentioned above, use can be made of a complex thereof.Specifically, selection is made of an n multiple of a single sort ofband, a mixture of different sorts of bands, and a combination thereof.In other words, time slots can be desiredly combined to be used as acut-through managing unit.

Referring to FIG. 6, a cut-through transmission apparatus according to athird embodiment of this invention will be described. In the figure, onetransmission path alone is illustrated.

The cut-through transmission apparatus of this embodiment comprises aSDH path VT1.5 time slot extracting section 41 for converting an inputoptical signal supplied through a point A into an input electric signaland for selecting among time slots in an input SDH path a particulartime slot including a packet to be dropped or to be added at a point B,a drop packet extracting section 42 for monitoring all packets in theparticular time slot selected by the SDH path VT1.5 time slot extractingsection 41 to identify whether or not each packet is to be dropped atthe point B, an add packet inserting section 43 for packet-multiplexinga packet not to be dropped at the point B and a packet to be inserted atthe point B to produce a packet-multiplexed packet, and an SDH signaltransmitting section 44 for inserting the packet-multiplexed packet tobe sent to a point C into an appropriate time slot, establishing anoutput SDH path, converting an output electric signal into an outputoptical signal, and delivering the output optical signal to the point C.

The cut-through transmission apparatus according to this embodiment isoperated as follows.

The input optical signal inserted through the point A is converted bythe SDH path VT1.5 time slot extracting section 41 into the inputelectric signal. Among the time slots in the SDH frame, selection ismade of the particular time slot including the packet to be dropped orto be added at the point B. In this embodiment, the time slot isextracted at a level of n x VT1.5 as a single path.

Each unselected time slot includes only those packets which need not bedropped at the point B (this node) or will include those packets to beadded at the point B (this node). The unselected time slot is sent fromthe SDH path VT1.5 time slot extracting section 41 to the SDH signaltransmitting section 44 and then from the SDH signal transmittingsection 44 to the point C.

When the unselected time slot is sent from the SDH path VT1.5 time slotextracting section 41 to the SDH signal transmitting section 44, theabove-mentioned cut-through 1 is carried out as illustrated in FIG. 6.

On the other hand, the selected time slot includes the packets to bedropped or will include the packets to be added. The drop packetextracting section 42 monitors all of the packets in the selected timeslot to identify whether or not each packet is to be dropped at thepoint B. If a particular packet is identified as a drop packet to bedropped at the point B, the drop packet is sent to the point B.

On the other hand, if the particular packet is identified as a non-droppacket not to be dropped at the point B, the non-drop packet is sentfrom the drop packet extracting section 42 to the add packet insertingsection 43. The add packet inserting section 43 packet-multiplexes thenon-drop packet with an add packet to be inserted at the point B toproduce a packet-multiplexed packet.

When the non-drop packet is sent from the drop packet extracting section42 to the add packet inserting section 43, the above-mentionedcut-through 2 is carried out as illustrated in FIG. 6.

The packet-multiplexed packet produced by the add packet insertingsection 43 is sent to the SDH signal transmitting section 44. The SDHsignal transmitting section 44 inserts the packet-multiplexed packetinto an appropriate time slot. After an output SDH frame is restructuredin the above-mentioned manner, the SDH signal transmitting section 44converts an output electric signal into an output optical signal anddelivers the output optical signal to the point C.

Referring to FIG. 7, a cut-through transmission apparatus according to afourth embodiment of this invention will be described. In thisembodiment, the cut-through transmission apparatus is applied to a ring.

In this embodiment, a first shared SDH path 51 is accessible at twopoints A and C. A second shared SDH path 52 is accessible at two pointsB and D.

For example, at each of the points A and C, the cut-through 2 and thecut-through 1 are applied to the first shared SDH path 51 and the secondshared SDH path 52, respectively. On the contrary, at the points B andD, the cut-through 1 and the cut-through 2 are applied to the firstshared SDH path 51 and the second shared SDH path 52, respectively.

In the example illustrated in FIG. 7, the number of nodes may beincreased. To each of additional nodes, the cut-through transmissionapparatus is applicable in the manner similar to the point A or B.

Referring to FIG. 8, the cut-through transmission apparatus of thefourth embodiment is applied to a multi-tenant building 60.

The multi-tenant building 60 is used by a plurality of users.Specifically, an underground node is arranged on a basement. XCorporation and Z Corporation have their offices on a first floor. On asecond floor, Y Corporation has its offices for two differentdepartments. On a third floor, Z Corporation and X Corporation havetheir offices. On a fourth floor, X Corporation has its office.

In this embodiment, the multi-tenant building 60 has a network whichcomprises a shared ring connecting the offices of the above-mentionedcorporations. In this ring, the underground node extracts acommunication band required by the multi-tenant building 60 as a whole.After the communication band is extracted, the underground node developscommunication to the ring for distribution to the offices of therespective corporations on the respective floors. Each office extracts apart of the communication required by the corporation and cuts throughthe remaining part of the communication for other corporations.

Thus, it is possible to reduce the functions of the node arranged ateach office and to suppress the packet delay.

For example, a part of communication for the X Corporation is cutthrough at the office of the Y Corporation. Therefore, the office of theY Corporation can not access to the part of communication for the XCorporation. Thus, the security is assured by executing the cut-through.

Furthermore, the X Corporation can communicate with its another officein an adjacent building through the underground node. Communication isalso possible between different offices of the X Corporation within thesame building.

According to this embodiment, a network cost per corporation can bereduced and management of the network is facilitated.

Referring to FIG. 9, an access network is applied to the above-mentionedring.

In this embodiment, the access network is developed from an internetbackbone 70 through an ADM (Add/Drop Multiplexer) 1 to a WAN 71 such asa metropolitan ring.

In the embodiment illustrated in FIG. 9, the WAN 71 as a whole has athroughput of 150 Mbps. The WAN 71 is connected through an ADM2 to athroughput of 50 Mbps for a first area 72 where residences and SOHOs areconcentrated, through an ADM3 to a throughput of 10 Mbps for a secondarea 73 where small sites are distributed, and through an ADM4 to athroughput of 100 Mbps for a third area 74 where high-rise buildings,molls, campuses are concentrated.

For example, in the first area 72, communication of the throughput of 50Mbps among 150 Mbps is carried out through the ADM2. For the remaining100 Mbps, no communication is performed.

Similarly, in the second area 73, communication of 10 Mbps is carriedout through the ADM3. In the third area 74, communication of 100 Mbps iscarried out through the ADM4. In other words, in the second area 73, theremaining throughput of 140 Mbps is cut through via the ADM3. In thethird area 74, the remaining throughput of 50 Mbps is cut through viathe ADM4.

Thus, it is possible to save the functions of each of ADMs 2, 3, and 4,to minimize the network delay as a whole, and to assure the security.

In each of the foregoing embodiments, the packets are dealt with. Thepackets are, for example, multiplexed in the add packet insertingsection 43 of the cut-through transmission apparatus illustrated in FIG.6. Alternatively, the packets may be replaced by cells which arecell-multiplexed. In this event also, the effect similar to thatobtained in the foregoing embodiments can be achieved.

In the foregoing embodiments, the SDH path is used. Alternatively, theSDH path may be replaced by a PDH (plesiochronous digital hierarchy)path or a WDM (wavelength division multiplexing) path.

As described above, in the cut-through transmission apparatus or thecut-through transmission method according to this invention, the signalto be hopped to the next node is cut through in the first or the secondlayer while only those packets to be dropped at this node are sent tothe third layer. Therefore, it is possible to considerably save theoperation in the third layer, i.e., the routing operation and to reducethe load imposed upon the router forming the third layer.

In case where the cut through is carried out in the second layer, thecut through is carried out without terminating the first layer.Therefore, the operation required to terminate the first layer (forexample, pointer processing upon terminating the VT1.5) is unnecessary.This makes it possible to considerably save the functions.

1. A node comprising first, second, and third layers, wherein: a cell is mapped in said first layer; said first layer judging whether the cell is to be dropped at said node or to be hopped to a next node; said first layer transmitting the cell to said third layer through said second layer when said first layer judges that the cell is to be dropped at said node.
 2. A node as claimed in claim 1, wherein said first layer transmits, when said first layer judges that the cell is to be hopped to said next node, the cell to said node by making the cell cut through said first layer.
 3. A transmission method carried out in a node comprising first, second, and third layers, comprising: judging, in said first layer where a cell is mapped, whether said cell is to be dropped at said node or to be hopped to a next node; and transmitting in said first layer the cell to said third layer through said second layer when said first layer judges that the cell is to be dropped at said node.
 4. A transmission method as claimed in claim 3, further comprising: transmitting in said first layer, when said first layer judges that the cell is to be hopped to said next node, the cell to said next node by making the cell cut through said first layer. 